Darren Rhodes wrote:
>
> On 06 Aug 98 11:26:36 , faurecm@halcyon.com (C. Marin Faure) wrote:
> >This is not true of turbofan engines, as long as temperature and N-speeds
> >are not exceeded. If you look at the graph representing the chances of
> >failure vs. power output of piston engines and turbofan engines, the
> >failure rate line climbs steeply with power output from a piston engine,
> >but remains relatively flat with turbofan engines.
>
> I have heard of a 747-400 operator that has 9 out of 10 engine
> shutdowns at high power, i.e. during takeoff.
>
> Temperature exceedance is one thing, but reduced margin at high power
> on high time turbine blades is an issue.
Of course the probability of failure is higher at maximum power. This is
true for any engine type I can think of.
I think the point that Mr. Faure was making is that the *rate* of
increasing probability of failure as a function of power setting is very
different for piston vs. turbine engines. A piston engine is
*measureably* more likely to fail at 80% power than it is at 70% power.
To see a comparable difference in probability of failure with a turbine
engine, you would have to compare operation at, say, 50% and 98% power.
A piston engine, despite its much earlier arrival on the scene, is
*vastly* more complex than a turbine engine. My experience is with
automotive piston engines, but many of the same principles apply to aero
engines. With racing auto engines, you *must* be concerned with things
like the fact that the cylinder block (although it may be a solid 300
pound chunk of cast iron) *flexes* at high power settings. The
crankshaft flexes, both in torsion and in lengthwise bending modes. The
thrust face of cylinder walls is actually distorted on each power
stroke, especially if the rod length to stroke ratio is less than about
1.8 (which is one of many reasons I think the ubiquitous small-block
Chevrolet is such a pile of junk compared to small block Ford and
Chrysler v8 engines, but thats another story for another newsgroup :-)
Valves bounce and stretch, cam lobes see in excess of 10,000 PSI at the
point they contact the lifter face (all the while the cam lobe is
rotating at up to 3500 RPM on a 7000 RPM engine), piston rings distort,
connecting rods stretch, and all *sorts* of other things happen even at
normal power settings, and it gets worse much faster with increasing
power. There is even a unique failure mode caused solely by a sudden
*reduction* of power output at high rpm. This just doesn't happen with a
turbine- there are a handful of failure modes that become critical at
high power, but that is *way* up on the power band- typically *beyond*
the power level that the remaining engine of a twin is called to produce
after the other engine fails.
I love the big piston aero engines, but only because they are such
incredible pieces of engineering. I would much rather trust my life to a
boring ol' turbine :-)
Stephen Lacker
Applied Research Laboratories, The University of Texas at Austin
slacker@arlut.utexxas.edu (Remove the extra 'x' to mail me)